Motor drive

ABSTRACT

A motor drive comprises a rectifier circuit portion arranged to receive an externally supplied AC voltage and to generate a DC bus voltage. An inverter circuit portion is arranged to receive the DC bus voltage (V DC_Bus ) and to generate an AC output voltage (V out ) for supply to an external load. A DC bus portion is connected between the rectifier and the inverter. An inductor (L 1 ) is connected in series along a bus conductor between the rectifier and inverter, and a DC link capacitor (C 1 ) is connected in parallel between the bus conductors. A voltage across the DC link capacitor (C 1 ) is input to a tuneable notch filter arranged to supply a filtered signal. A controller varies the resonant frequency of the notch filter to a plurality of values across an operational range and modulates a supply current provided by the inverter with a probe current signal at the resonant frequency.

FOREIGN PRIORITY

This application claims priority to European Patent Application No.20275040.2 filed Feb. 12, 2020, the entire contents of which isincorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to a motor drive arranged to monitor degradationof an internal capacitor, in particular the capacitance of a DC linkcapacitor of the motor drive.

BACKGROUND ART

Typical industrial motor drives include a front-end rectifier arrangedto convert an externally supplied AC voltage to a rectified DC voltage,and a power inverter that converts this rectified DC voltage to an ACvoltage suitable for supply to a motor. The motor may, in turn, drive anactuator. Such motor drives are used in a wide variety of applications,however some such motor drives are particularly applicable to aerospaceapplications.

The front-end rectifier and the power inverter are generally separatedby a DC bus, where the DC bus includes a DC link capacitor. The DC linkcapacitor is typically connected in parallel between the positive andnegative conductors of the DC bus and serves to smooth the DC voltageoutput of the front-end rectifier. The DC link capacitor also protectsupstream circuits from the transient response of downstream circuits.

Typically, the DC link capacitor value is selected to optimise thestability of the DC voltage across the bus. However, the capacitance ofthe DC link capacitor can change over time. One cause of this change incapacitance is a slow aging mechanism which leads to the capacitancedecreasing by a few percentage points over a relatively long period oftime, e.g. over a few years. A further cause is a failure mechanisminvolving dielectric breakdown followed by capacitor self-healing, whichcan cause sudden and significant drops in the capacitance.

In motor drives for aerospace applications, the weight and volume of themotor drive is an important consideration, where the weight and volumeare ideally minimal. The DC link capacitor is generally one of thelargest components of the motor drive by volume. In order to minimiseweight and volume, the minimal capacitance component possible istypically selected, however this generally means that there is littleovervoltage margin and little current ripple margin in the design whichmakes premature aging of the device more likely.

The present disclosure is concerned with providing an improved motordrive capable of detecting capacitance changes such that replacement ormaintenance can be carried out at the appropriate time while allowingthe use of a physically small and/or lightweight DC link capacitor.

SUMMARY OF THE DISCLOSURE

In accordance with a first aspect, the present disclosure provides amotor drive comprising: a rectifier circuit portion arranged to receivean externally supplied AC voltage and to generate a DC bus voltagetherefrom; an inverter circuit portion arranged to receive the DC busvoltage and to generate an AC output voltage therefrom for supply to anexternal load; a DC bus portion connected between said rectifier andinverter circuit portions, said DC bus portion comprising first andsecond conductors, wherein an inductor is connected in series along thefirst conductor between said rectifier and inverter circuit portions,and wherein a DC link capacitor is connected in parallel between thefirst and second conductors; a notch filter having a tuneable resonantfrequency, wherein a voltage across the DC link capacitor is input tothe notch filter, the notch filter being arranged to supply a filteredsignal; and a controller arranged to vary the resonant frequency of thenotch filter and to modulate a supply current provided by the invertercircuit portion with a probe current signal at the resonant frequency;wherein the controller is arranged to vary the resonant frequency to aplurality of values across an operational range, to measure an amplitudeof the filtered signal, to determine which of the plurality of values ofthe resonant frequency has a maximal amplitude of the filtered signalassociated therewith, and to determine a capacitance value of the DClink capacitor from said value of the resonant frequency associated withthe maximal amplitude.

The first aspect of the disclosure extends to a method of monitoringdegradation of a capacitor of a motor drive, the method comprising:exciting a DC bus with a current controlled by a power inverter;determining a shift in a natural frequency of a resonator on the DC bus,said resonator comprising an inductor and the capacitor; and attributingthe shift in natural frequency to a change in a capacitance of thecapacitor being outside of a selected threshold range of capacitancevalues. In some examples, the method of monitoring degradation of thecapacitor of a motor drive without using any additional components ofthe motor drive than those already present. The current controller bythe power inverter is, at least in some examples, a low amplitudecurrent.

This first aspect of the disclosure extends to a method of operating amotor drive comprising: a rectifier circuit portion and an invertercircuit portion having a DC bus portion connected therebetween, said DCbus portion comprising first and second conductors, wherein an inductoris connected in series along the first conductor between said rectifierand inverter circuit portions, and wherein a DC link capacitor isconnected in parallel between the first and second conductors; and anotch filter having a tuneable resonant frequency, wherein a voltageacross the DC link capacitor is input to the notch filter, the notchfilter being arranged to supply a filtered signal; wherein the methodcomprises: receiving an externally supplied AC voltage and to generate aDC bus voltage therefrom using the rectifier circuit portion; receivingthe DC bus voltage and generating an AC output voltage therefrom usingthe inverter circuit portion; supplying the AC output voltage to anexternal load; varying the resonant frequency of the notch frequency;modulating a supply current provided by the inverter circuit portionwith a probe current signal at the resonant frequency; varying theresonant frequency to a plurality of values across an operational range;measuring an amplitude of the filtered signal; determining which of theplurality of values of the resonant frequency has a maximal amplitude ofthe filtered signal associated therewith; and determining a capacitancevalue of the DC link capacitor from said value of the resonant frequencyassociated with the maximal amplitude.

Thus it will be appreciated that aspects of the present disclosureprovide an improved motor drive system and method of operating the samein which the capacitance of the DC link capacitor is determined byperforming a frequency sweep to probe the resonant frequency of theeffective resonant circuit constructed from the inductor and DC linkcapacitor, often referred to in the art as an ‘LC resonant circuit’ or‘LC resonator’.

It will be appreciated that the notch filter may not actually beconstructed from discrete physical components, and may be implementedusing logical function blocks, e.g. those already present in aconventional motor drive, e.g. logical function blocks of an existingcontroller, e.g. the controller that is arranged to vary the resonantfrequency of the notch filter. Other components of the motor drivesystem may similarly be implemented using logical function blocks ratherthan discrete hardware as appropriate.

By augmenting the current supplied to the inverter downstream of the LCresonator, the voltage across the DC link capacitor (which wouldotherwise have simply been a relatively steady-state DC voltage) isprovided with a ‘small signal’ (i.e. AC) component. By varying thefrequency of this AC component, the magnitude of the voltage across theDC link capacitor can be observed to determine the approximate resonantfrequency of the LC resonator, which in turn provides a measure of thecapacitance of the DC link capacitor as outlined in further detailbelow.

In general, the inductor is already present in the DC bus ofconventional motor drives known in the art per se. Thus, advantageouslyno additional hardware is required in order to form the LC resonator.The inductor itself is generally a fixed component, the inductance ofwhich does not generally vary over time.

As the inductor and DC link capacitor form an LC resonator, the resonantfrequency of the LC resonator is expressed as per Equation 1 below:

$\begin{matrix}{{f_{res} = \frac{1}{2\pi\sqrt{LC}}}{{Resonant}{}{frequency}{of}{an}{LC}{resonator}}} & {{Equation}1}\end{matrix}$where f_(res) is the resonant frequency of the LC resonator, L is theinductance of the inductor, and C is the capacitance of the DC linkcapacitor.

By sweeping through a number of different candidate frequency valuesacross the operational range and selecting the value leading to thelargest amplitude at the output of the notch filter, an approximation ofthe resonant frequency f_(res) of the LC resonator can be determined.Additionally, because the inductance L of the inductor is known, thecapacitance C of the DC link capacitor may be determined by rearrangingEquation 1 as per Equation 2 below:

$\begin{matrix}{{C = \left( \frac{1}{2\pi f\sqrt{L}} \right)^{2}}{{Capacitance}{}{of}{the}{DC}{link}{capacitor}{within}{an}}{{LC}{resonator}}} & {{Equation}2}\end{matrix}$

The typical motor phase currents may vary between approximately 1 A and100 A, depending on the exact nature of the application, e.g.electromechanical actuators, fans, pumps, thrust reversers, electricbrakes etc. The frequency of the phase currents is, in general,proportional to the speed of the motor. In a typical application, thefrequency increases from 0 Hz when the motor starts spinning and canreach up to approximately 1 kHz, depending on the top speed andconstruction of the motor. The amplitude of the motor phase voltage isgenerally dependent on the motor current and the motor speed. Typically,the amplitude increases from just a few volts at zero speed topotentially several hundred volts at top speed. The inverter ideallysupplies power with the correct voltage amplitude and frequency to themotor, where the motor current is dictated by Ohm's law. The voltagedemand may be calculated by a digital motor controller based on therequired motor current which, in turn, may depend on the motor speederror, i.e. a difference between a reference speed and an actualmeasured speed of the motor.

The DC link passive components—e.g. the capacitor and inductor(s)—may beselected such that the resonant frequency is above the maximum frequencyof the motor phase currents and voltages. The resulting resonantfrequency may therefore be somewhere between the top motor phase voltagefrequency and the inverter PWM frequency, and may for example be betweenapproximately 1 kHz and 10 kHz.

The probe current signal referred to hereinabove may be a ‘currentdemand’ with frequency in the order of approximately a few kHz but witha relatively low amplitude (e.g. below approximately 1 A). This currentdemand may be translated into an appropriate voltage demand by thecontroller (e.g. a digital motor controller). This may then be PWMmodulated and applied to the inverter power transistors. Compared toconventional arrangements known in the art per se, the probe currentsignal has a relatively low amplitude but relatively high frequency,whereas ‘normal’ motor control signals generally have much higheramplitude and much lower frequency.

The controller may carry out the test of the capacitance value of the DClink capacitor at any time, however for simplicity it may beadvantageous to carry out the tests when the external load is idle, e.g.if a motor connected downstream of the inverter is idle or powered off.In some examples, the controller performs the frequency sweep todetermine the capacitance value during a start-up procedure of the motordrive. The ‘front-end’ rectifier may not allow the current on the DClink to change polarity during the resonant LC operation mode. As such,resonance can only take place if there is a minimum level of dc currenton the DC link, such that the total current (i.e. the dc ‘steady state’component plus the resonant component) does not cross zero. Thiscondition may be achieved during what is known as ‘pre-charge’ of the DClink. Those skilled in the art will appreciate that this is the timebetween the initial connection to the external power supply and the timewhen the DC link capacitor has reached its nominal voltage. Thepre-charge current typically ramps up rapidly from zero to a peak valuedependent on the size of a pre-charge resistor and then it decaysexponentially back to zero. In some examples, the capacitor test can beperformed during the exponential decay of the pre-charge current. Thus,in accordance with examples of the present disclosure, the controllerprovides a ‘Power-up Built-In Test’ (PBIT) mechanism.

If the determined capacitance of the DC link capacitor has strayed fromits intended value, e.g. by more than a threshold amount which may beset in accordance with an acceptable tolerance, in some examples analert may be raised by the controller. This alert may therefore beraised if the instantaneous capacitance value at the time of measurementis sufficiently far removed from the acceptable capacitance value. Thealert may simply comprise a warning flag (e.g. a register value may betoggled from ‘0’ to ‘1’), however in some examples the alert maycomprise further information regarding the capacitance degradation, e.g.the most recently determined value of the capacitance of the DC linkcapacitor and/or a rate of change of the capacitance as outlined below.

The first and second conductors may, at least in some examples, bepositive and negative rails (or ‘busbars’) respectively, such that theinductor is connected in series along the positive DC rail. However,other examples are envisaged in which the inductor is connected inseries along the negative DC rail. Additionally or alternatively, aninductor may be connected in series along each of the first and secondconductors, e.g. there may be an inductor connected along each of thepositive and negative rails. Where each of the positive and negativerails or busbars is provided with an inductor, these inductors may bemagnetically coupled in some examples, however in other examples theinductors are not magnetically coupled.

In some examples, the controller is arranged to compare the determinedcapacitance value to a stored capacitance value and determine adifference between said determined and stored capacitance values. In aset of examples, the motor drive comprises a memory, for example anon-volatile memory (NVM), where the stored capacitance values are held.The controller may, in some examples, be arranged to store thedetermined capacitance value in the memory and/or to retrieve the storedcapacitance value from memory.

In some examples, the controller is arranged to compare the determinedcapacitance value to a plurality of stored capacitance values or astored capacitance trend. Thus, in such examples, the controller maydetermine the rate at which the capacitance of the DC link capacitor ischanging and determine whether replacement or maintenance is requiredand, if necessary, raise an alert.

While the input of the notch filter could be directly connected betweenthe inductor and DC link capacitor, in some examples the DC bus portioncomprises a voltage sensor arranged to produce a sense signal dependenton a voltage between the inductor and the DC link capacitor. Such asensor may help to determine when the capacitor has reached full voltageduring the power up sequence, and/or it may help to detect overvoltagesituations during the operation of the motor in regenerative mode. Itwill be appreciated that there are various voltage sensors that may beused, however in some examples, the voltage sensor comprises a potentialdivider connected between the inductor and DC link capacitor, whereinthe node is connected between first and second resistors of thepotential divider. In some such examples, the potential divider isconnected in parallel across the first and second conductors. Thispotential divider-based voltage sensor may already be a componentprovided as part of (or alongside) a conventional controller and thusmay advantageously not require any further hardware. The voltage sensormay also include an isolation amplifier and/or an ADC.

Those skilled in the art will appreciate that there are a number oftuneable notch filters, known in the art per se, that could be used toimplement the motor drive disclosed herein. However, in some examples,the notch filter comprises: a first low pass filter block arranged toremove a DC component of a voltage at the input of the notch filter toproduce an AC component; a forward rotation vector block arranged togenerate a first vector having a first element set to zero and a secondelement set to the AC component, and to recalculate said vector in areference frame rotating at the resonant frequency of the notch filter,thereby generating a second vector; a second low pass filter blockarranged to filter a first element and a second element of secondvector, thereby generating a third vector; and a backward rotationvector block arranged to recalculate the third vector in a stationaryreference frame, thereby generating a fourth vector; wherein a secondelement of the fourth vector is output as the filtered signal.

This is novel and inventive in its own right and thus, when viewed froma second aspect, the present disclosure provides a notch filtercomprising: a first low pass filter block arranged to remove a DCcomponent of a voltage at the input of the notch filter to produce an ACcomponent; a forward rotation vector block arranged to generate a firstvector having an first element set to zero and a second element set tothe AC component, and to recalculate said vector in a reference framerotating at a resonant frequency of the notch filter, thereby generatinga second vector; a second low pass filter block arranged to filter afirst element and a second element of second vector, thereby generatinga third vector; and a backward rotation vector block arranged torecalculate the third vector in a stationary reference frame, therebygenerating a fourth vector; wherein a second element of the fourthvector is provided at an output of the notch filter.

This second aspect of the present disclosure extends to a method offiltering an input voltage, the method comprising: removing a DCcomponent of the input voltage to produce an AC component; generating afirst vector having a first element set to zero and a second element setto the AC component; recalculating said first vector in a referenceframe rotating at a resonant frequency of the notch filter, therebygenerating a second vector; low-pass filtering a first element and asecond element of second vector, thereby generating a third vector;recalculating the third vector in a stationary reference frame, therebygenerating a fourth vector; and providing a second element of the fourthvector as an output.

It will be appreciated that the term ‘low pass filter’ means a filterthat substantially removes components having a frequency greater than acorresponding cut-off frequency of the filter, but substantially allowscomponents having a frequency less than the cut-off frequency of thefilter.

In some examples, the first, second, third, and fourth vectors aretwo-dimensional vectors. The first element of each vector may be anx-component and the second element of each vector may be a y-component.However, the terms ‘x-component’ and ‘y-component’ are not intended tolimit the scope to a particular parameterisation in which the ‘upper’element of the first vector is set to zero and the ‘lower’ element isset to the AC component, as these roles may be readily exchanged withoutdeviating from the scope of the invention by rearranging the rotationvectors applied by the vector blocks.

In some examples the external load comprises a motor. In some suchexamples, the motor is arranged to drive an actuator, a fan, and/or apump, e.g. an oil pump for a hydraulic system.

In some examples, the supply current provided by the inverter isgenerated by a current control loop arrangement, wherein the currentcontrol loop arrangement is arranged to receive a modulation signal fromthe controller. This current control loop arrangement may comprise a‘cascaded control loop’. Such a cascaded control loop may include aplurality of control loops embedded inside each other in accordance withthe principles of ‘cascade control’, known in the art per se. In brief,each loop calculates the set-point for the next loop. By way ofnon-limiting example only, the outermost loop may comprise a speedcontrol loop which receives a speed reference from an external commandand calculates a speed error as a difference between the speed referenceand a measured speed. This speed error may drive a proportional-integral(PI) controller that calculates a motor current reference which may thenbe used as the set point of the inner current control loop. Similarly, acurrent error may calculated as a difference between the current setpoint and a measured motor current. This, in turn, may drive a PIcontroller that generates a motor voltage reference, which may then bePWM modulated and transmitted to the power inverter.

It will be appreciated that the optional features described in respectof examples of any aspect of the present disclosure also apply to allother aspects of the present disclosure as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain examples of the present disclosure will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a motor drive system in accordance withan example of the present disclosure;

FIG. 2 is a block diagram of a notch filter for use in the motor drivesystem of FIG. 1;

FIG. 3 is a graph of a DC voltage vector in the rotating referenceframe;

FIG. 4 is a graph of a filtered trajectory of the vector of FIG. 3;

FIG. 5 is a graph of the filtered trajectory of FIG. 4 translated backto a stationary reference frame;

FIG. 6 is a graph of the transient filter response;

FIG. 7 is a graph of the steady-state filter response;

FIG. 8 is a block diagram of the capacitance monitor portion used in themotor drive system of FIG. 1;

FIGS. 9A and 9B are vector plots illustrating a Clarke transform;

FIGS. 10A and 10B are vector plots illustrating a Park transform; and

FIG. 11 is a graph illustrating a PWM modulation scheme.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of a motor drive system 2 in accordancewith an example of the present disclosure. The motor drive system 2includes a front-end rectifier 4 and a power inverter 6, which areconnected by a DC bus 8.

The front-end rectifier 4 is arranged to receive an externally suppliedAC voltage Vsupply, which in this example is a three-phase AC inputvoltage. The rectifier 4 converts this AC voltage Vsupply to a DC busvoltage VDC_Bus which is transferred to the inverter 6 across the DC bus8, the details of which are discussed in more detail below. It will beappreciated that other arrangements are possible, e.g. in which therectifier 4 receives a single-phase input. Thus the rectifier 4 is anAC-to-DC converter (ADC).

The inverter 6 takes the DC bus voltage VDC_Bus and converts it back toan AC output voltage Vout suitable for supply to the connected load. Inthis example, the inverter 6 supplies the output voltage Vout to a motor10, which in turn is arranged to drive an actuator 12. In this example,the output voltage Vout provided to the motor 10 is a three-phasevoltage suitable to drive the three-phase motor 10. It will beappreciated that other arrangements are possible, e.g. in which theinverter 6 produces a single-phase output. Thus the inverter 6 is aDC-to-AC converter (DAC).

The DC bus 8 includes a protective arrangement 14 constructed from aresistor R1 connected in parallel with a thyristor D1. This resistor R1serves to limit the inrush current during DC link pre-charge immediatelyafter the input power supply is switched on. The thyristor D1 provides abypass around the resistor R1 after the end of the pre-charge in orderto limit power losses of the motor drive system 2.

The output of the protective arrangement 14 is connected to a terminalof an inductor L1 which is arranged in series along the positiveconductor 16 of the DC bus 8. The other terminal of the inductor L1 isconnected to the input of the inverter 6.

A DC link capacitor C1 is connected in parallel between the positiveconductor 16 and the negative conductor 17 of the DC bus 8. This DC linkcapacitor C1 serves to smooth the DC bus voltage VDC_Bus output of thefront-end rectifier 4. The DC link capacitor also protects upstreamcircuits from the transient response of downstream circuits.

A potential divider is connected in parallel to the DC link capacitor C1between the positive conductor 16 and the negative conductor 17 of theDC bus 8. The potential divider is constructed from a pair of resistorsR2, R3, having a node 18 connected between them. This node 18 isconnected to a capacitance monitor portion 20, as explained in furtherdetail below.

A pair of transistors T1, T2 and a resistor R4 are provided foroperation of the motor in its ‘regenerative mode’ which may, forexample, be used when the motor is working to slow down the mechanicalactuator. In this case, kinetic energy is converted to electrical energywhich is transferred to the capacitor as electrical charge. In otherwords, the motor operates as a generator in such situations. In order toprotect the system from the capacitor voltage increasing to potentiallydangerously high levels, the two additional transistors T1, T2 areswitched on to allow the excess capacitor charge to discharge across theso-called ‘brake resistor’ R4.

The capacitance monitor portion 20 includes a DC Voltage ADC interface22, a tuneable notch filter 24, an PBIT controller 26, an NVM interface28, and a current control loop 30. It will be appreciated that the‘capacitance monitor portion’ 20 is a collective term for the componentsused to detect degradation of the DC link capacitor C1 and is used forthe purposes of explanation only. Some or all of the components in thisportion 20 may be wholly separate components (i.e. physically distinct),or may be integrated within the same component, e.g. implemented asfunctions of a microprocessor, application specific integrated circuit(ASIC), field-programmable gate array (FPGA), etc., as appropriate. Inpreferred examples, the use of discrete physical components is minimisedand ideally avoided in order to reduce the cost and complexity of thesystem.

The DC Voltage ADC interface 22 is connected to the node 18 between thepotential divider resistors R2, R3, and thus receives a voltage that isproportional to the voltage at the node 32 between the inductor L1 andthe DC link capacitor C1, in accordance with the ratio between theresistances of the potential divider resistors R2, R3. This DC VoltageADC interface 22 converts the analogue voltage at the node 18 to adigital value supplied to the notch filter 24. In practice, this DCVoltage ADC interface 22 may form part of the notch filter 24 or may bean existing logic block within the motor drive, however it is shown as adiscrete block in FIG. 1 for ease of reference. This ADC interface 22may, in general, be an existing ADC interface already present within anotherwise-conventional motor drive that can be repurposed for thepurposes of the present disclosure.

The notch filter 24 is tuneable across an operating range of frequenciesaround the nominal resonant frequency of the LC circuit comprising theinductor L1 and the DC link capacitor C1, assuming no degradation of theDC link capacitor C1. The resonant frequency of the notch filter 24 iscontrolled in steps by the PBIT controller 26 as explained in furtherdetail below. Essentially, the notch filter 24 is a highly selectivefilter (i.e. it has a relatively narrow pass band, potentially of onlyseveral Hertz) that outputs a signal having a magnitude dependent on themagnitude of frequency components of the signal at its input that fallwithin the notch filter's pass band.

The PBIT controller 26 receives the output of the notch filter 24 andcontrols the frequency sweep-based test of the motor drive 2. Thecontroller 26 provides a modulating, sinusoidal AC current to thecurrent control loop 30, which uses this signal to determine a voltagedemand which is subsequently PWM modulated and provided to the inverter6. The current control loop 30 modulates a steady state current ID that,in this example, is the steady state D-axis current ID* supplied to themotor 10.

The provision of PWM switching patterns provided to the inverter 6 givesrise to an alternating voltage across the motor and a correspondingalternating current the DC link capacitor C1, i.e. it indirectlysupplies an AC input to the LC resonator within the DC bus 8. As aresult, the magnitude of the voltage at the node 18 will vary inresponse to this indirectly injected AC input to the LC resonator. Themagnitude of the voltage at the node 18 will depend on the capacitanceof the DC link capacitor C1, which will in turn affect the cut-offfrequency of the LC resonator.

The PBIT controller 26 steps through a number of different resonantfrequency values, wherein for each step, the frequency of the modulatingcurrent Imod applied to the current control loop 30 and the tunedfrequency of the notch filter 24 are updated to the same value.

The magnitude of the output of the notch filter 24 is observed by thePBIT controller 26 for each frequency step and may be stored in a NVM 34via the NVM interface 28. While sweeping through the frequencies, thePBIT controller 26 determines which of the frequency settings gives riseto the greatest magnitude at the output of the notch filter 24, therebydetermining the approximate resonant frequency of the LC resonatorwithin the DC bus 8.

Once the approximate resonant frequency of the LC resonator within theDC bus 8 is determined, the PBIT controller 26 may then determine thecapacitance of the DC link capacitor C1 using the relationship describedpreviously with reference to Equation 2. Depending on the determinedcapacitance value, the PBIT controller 26 may raise an alert (e.g. ifthe capacitance value has deviated from its intended value by more thana threshold amount) indicating that replacement or repair of the DC linkcapacitor C1 (or if necessary part or all of the motor drive 2) isrequired.

The PBIT controller 26 may also compared the determined capacitancevalue to one or more previously determined capacitance values stored inthe NVM 34 to determine a rate of change of the capacitance and todetermine approximately when replacement or repair may be necessary,even if it is not currently required.

The construction of a suitable notch filter 24 can be seen in FIG. 2,and the operation of this filter 24 may be understood with reference toFIGS. 3 to 7.

The notch filter 24 comprises a first low pass filter block 36 that isarranged to receive the voltage from the node 18 between the resistorsR2, R3 of the potential divider via the DC Voltage ADC interface 22.This low pass filter block 36 produces an ‘average’ Vdc of the voltagepresented at its input (i.e. the DC component of the voltage from thenode 18), which is then subtracted from the voltage from the node 18,i.e. the voltage Vcap across DC link capacitor C1, thereby leaving onlythe AC component of the voltage from the node 18.

A forward rotation vector block 38 is arranged to generate a firstvector

$\begin{pmatrix}0 \\{V_{cap} - V_{dc}}\end{pmatrix}$having an x-coordinate set to zero and a y-coordinate set to the ACcomponent resulting from the subtraction of the average Vdc from thevoltage Vcap across DC link capacitor C1.

This forward rotation vector block 38 then recalculates this vector in areference frame rotating at the resonant frequency of the notch filter,thereby generating a second vector, in accordance with Equation 3:

$\begin{matrix}{{{{}\begin{pmatrix}V_{x}^{rot} \\V_{y}^{rot}\end{pmatrix}} = {\begin{pmatrix}{\cos\omega t} & {\sin\omega t} \\{{- \sin}\omega t} & {\cos\omega t}\end{pmatrix}\begin{pmatrix}0 \\{V_{cap} - V_{dc}}\end{pmatrix}}}{{Rotation}{}{performed}{by}{the}{forward}{rotation}}{{vector}{block}38}} & {{Equation}3}\end{matrix}$

The trajectory of this vector

$\begin{pmatrix}V_{x}^{rot} \\V_{y}^{rot}\end{pmatrix}$can be seen in FIG. 3, which is a graph of the voltage vector in therotating reference frame. The ‘flower-like’ pattern that can be seen isa result of the rotating frame of reference. It will be appreciated thatthe angular frequency ω referred to with reference to FIGS. 3 to 7 isthe angular frequency corresponding to the current resonant frequencyvalue f as set by the PBIT controller 26, and may be calculated asω=2πf.

A second low pass filter block 39—which in this implementation isconstructed from two separate low pass filters 40, 42 but could comprisea single filter block in other examples—is arranged to filter the x- andy-components of the second vector, i.e. the output of the forwardrotation vector block 38, thereby generating a third vector

${\begin{pmatrix}V_{x}^{filt} \\V_{y}^{filt}\end{pmatrix}.}$The trajectory of this third filtered vector can be seen in FIG. 4. Itshould be noted that the filters 40, 42 attenuate the magnitude of thevector by half, however this is compensated for later as outlined below.

A backward rotation vector block 44 is arranged to recalculate the thirdvector in the stationary reference frame, thereby generating a fourthvector, in accordance with Equation 4:

$\begin{matrix}{{{\begin{pmatrix}V_{x}^{stat} \\V_{y}^{stat}\end{pmatrix} = {\begin{pmatrix}{\cos\omega t} & {{- s}{in}\omega t} \\{\sin\omega t} & {\cos\omega t}\end{pmatrix}\begin{pmatrix}V_{x}^{filt} \\V_{y}^{filt}\end{pmatrix}}}{Rotation}{}{performed}{by}{}{the}{backward}{rotation}}{{vector}{block}44}} & {{Equation}4}\end{matrix}$

The trajectory of this vector

$\begin{pmatrix}V_{x}^{stat} \\V_{y}^{stat}\end{pmatrix}$can be seen in FIG. 5, which is a graph of the voltage vector in thestation reference frame. The ‘flower-like’ pattern that can be seen is aresult of the rotating frame of reference.

The y-component V_(y) ^(stat) of the fourth vector is output as thefiltered signal, i.e. the resonant voltage amplitude for the currentfrequency as set by the PBIT controller 26 is extracted from V_(y)^(stat) Finally an amplifier 46 multiplies the magnitude of the V_(y)^(stat) by two (i.e. it doubles the magnitude) in order to compensatefor the attenuation referred to above.

The length of the transient response increases with decreasing filterbandwidth. The transient response can be adjusted by selecting the orderof the low-pass filters 40, 42. Where first order low pass filters areused, these may provide transient responses in the order of tens ofmilliseconds for a bandwidth of 20 Hz as illustrated by FIG. 6. Thesteady-state filter response can be seen in FIG. 7.

A shorter transient time can be achieved with second order low-passfilters, where mathematical complexity is traded off for a fasterresponse time.

Those skilled in the art will readily appreciate the respectivefunctions of the other components of the capacitance monitor portion 20.However, for reference, the capacitance monitor portion 20 also includesa phase current ADC interface 48; a Clarke transform block 50; a Parktransform block 52. The structure of these elements may be understoodwith reference to FIG. 8, in which some of the functional blocks arebroken down into simpler functional blocks for ease of reference.

The phase current ADC interface 48 is a functional block which reads thevalues of the three motor phase currents from A/D Converters connectedto analogue current sensors.

The Clarke transform block 50 is a functional block which calculates aso-called ‘stator current space vector’. Those skilled in the art willappreciate that this ‘space vector’ is a generally understood term inthe field of motor control theory which applies to any set of threequantities such as three phase currents, three phase voltages, threemagnetic fluxes, etc. The original quantities (currents, voltages, ormagnetic fluxes as appropriate) are scalar but they are associated withthree basis vectors arranged 120° apart around the origin of atwo-dimensional space. Positive scalars are transformed into vectors inthe direction of the associated basis vector, whereas negative scalarsproduce opposite vectors. The final ‘space vector’ is the sum of thesethree scalars-converted-to-vectors, as can be seen in FIG. 9A.

The space vector is algebraically represented in a coordinate system (α,β), as can be seen in FIG. 9B. The vector is also rescaled such that thelength of the space vector is equal to the amplitude of the motor phasecurrents or voltages or fluxes. The resulting equations for the Clarketransform are given as per Equation 5 below:

$\begin{matrix}\left\{ {\begin{matrix}{I_{\alpha} = I_{a}} \\{I_{\beta} = \frac{I_{b} - I_{c}}{\sqrt{3}}}\end{matrix}{Clarke}{Transform}} \right. & {{Equation}5}\end{matrix}$

The Park transform block 52 is a functional block which recalculates thecurrent space vector from the (α, β) coordinates into (d, q) coordinatesystem which is aligned to the magnetic field of the rotor and itrotates synchronously with the rotor, as can be seen in FIGS. 10A and10B, where the equations for the Park transform are given as perEquation 6 below:

$\begin{matrix}{{\begin{pmatrix}I_{d} \\I_{q}\end{pmatrix} = {\begin{pmatrix}{\cos\theta} & {\sin\theta} \\{{- s}{in}\theta} & {\cos\theta}\end{pmatrix}\begin{pmatrix}I_{\alpha} \\I_{\beta}\end{pmatrix}}}{{Park}{Transform}}} & {{Equation}6}\end{matrix}$

The current control loops 30 include two PI controllers 54, 56. Thefirst PI controller 54 is arranged to control the d-axis currentcomponent I_(d) and the other PI controller 56 is arranged to controlthe q-axis current component I_(q). These PI controllers 54, 56determine the appropriate current error (i.e. the difference between setpoint and measured current) and produce corresponding voltage demandsV_(d_dem) and V_(q_dem) respectively.

An Inverse Park transform block 58 changes the coordinates of voltagespace vector (V_(d_dem), V_(q_dem)) to (V_(α_dem), V_(β_dem)) as perEquation 7 below:

$\begin{matrix}{{\begin{pmatrix}I_{\alpha}^{dem} \\I_{\beta}^{dem}\end{pmatrix} = {\begin{pmatrix}{\cos\theta} & {{- s}{in}\theta} \\{\sin\theta} & {\cos\theta}\end{pmatrix}\begin{pmatrix}I_{d}^{dem} \\I_{q}^{dem}\end{pmatrix}}}{{Inverse}{Park}{Transform}}} & {{Equation}7}\end{matrix}$

An Inverse Clarke transform block 60 converts the space vector(V_(α_dem), V_(β_dem)) to individual phase voltage demands V_(a_dem),V_(b_dem), V_(c_dem) as per Equation 8 below:

$\begin{matrix}{\left\{ {\begin{matrix}{V_{a}^{dem} = V_{\alpha}^{dem}} \\{V_{b}^{dem} = {{- \frac{V_{\alpha}^{dem}}{2}} + {\frac{\sqrt{3}}{2}V_{\beta}^{dem}}}} \\{V_{c}^{dem} = {{- \frac{V_{\alpha}^{dem}}{2}} - {\frac{\sqrt{3}}{2}V_{\beta}^{dem}}}}\end{matrix}{Inverse}{Clarke}{Transform}} \right.} & {{Equation}8}\end{matrix}$

A PWM modulator 62 transforms the individual phase voltage demands intopulse-width modulation patterns, where an illustrative example of abasic PWM modulation scheme can be seen in FIG. 11.

Also provided is a resolver interface 64 which is a functional blockthat calculates motor speed and rotor angular position based on thefeedback signals provided by a position sensor, in this case a resolver66 (which is connected to the interface 64 via an ADC interface 68). Thetwo feedback signals are typically named ‘Sin’ and ‘Cos’ because thecorresponding signals are amplitude-modulated signals with amplitudesproportional to the sine and the cosine of the rotor angle. The resolverinterface 64 performs the demodulation of the resolver feedback signalsto extract speed and position information. The resolver interface 64 isconnected to the ADC 68 which receive the analogue Sin and Cos signalsfrom the resolver 66.

Thus it will be appreciated that examples of the present disclosureprovide an improved motor drive system and method of operating the samein which the capacitance of the DC link capacitor is determined byperforming a frequency sweep to probe the resonant frequency of theeffective resonant circuit and thereby determine the current level ofdegradation of the DC link capacitor. In some examples, dangerouscapacitance drops due to dielectric breakdown may be detected veryearly, and potentially immediately. Also provided herein is a novelnotch filter and associated method.

The logical functions used by the components (e.g. the filtering androtation functions used by the notch filter) may, in some examples,already be otherwise present in a conventional motor drive and thus mayadvantageously require no additional hardware to implement. The presentdisclosure may therefore provide yet further volume and weight savingswhich is advantageous for e.g. aerospace applications.

Extraction of general capacitor degradation trends across all units of awhole aircraft fleet may be achieved, which may be useful for analysingreliability and in-service problems. Furthermore, early detection ofcapacitor degradation may help to schedule maintenance operations forparticular units.

While specific examples of the disclosure have been described in detail,it will be appreciated by those skilled in the art that the examplesdescribed in detail are not limiting on the scope of the disclosure.

The invention claimed is:
 1. A motor drive comprising: a rectifiercircuit portion arranged to receive an externally supplied AC voltageand to generate a DC bus voltage therefrom; an inverter circuit portionarranged to receive the DC bus voltage and to generate an AC outputvoltage therefrom for supply to an external load; a DC bus portionconnected between said rectifier and inverter circuit portions, said DCbus portion comprising first and second conductors, wherein an inductoris connected in series along the first conductor between said rectifierand inverter circuit portions, and wherein a DC link capacitor isconnected in parallel between the first and second conductors; a notchfilter having a tuneable resonant frequency, wherein a voltage acrossthe DC link capacitor is input to the notch filter, the notch filterbeing arranged to supply a filtered signal; and a controller arranged tovary the resonant frequency of the notch filter and to modulate a supplycurrent provided by the inverter circuit portion with a probe currentsignal at the resonant frequency; wherein the controller is arranged tovary the resonant frequency to a plurality of values across anoperational range, to measure an amplitude of the filtered signal, todetermine which of the plurality of values of the resonant frequency hasa maximal amplitude of the filtered signal associated therewith, and todetermine a capacitance value of the DC link capacitor from said valueof the resonant frequency associated with the maximal amplitude.
 2. Themotor drive as claimed in claim 1, wherein the controller determines thecapacitance value during a power-up procedure of the motor drive.
 3. Themotor drive as claimed in claim 1, wherein the controller generates analert when the determined capacitance of the DC link capacitor differsfrom a target value by more than a threshold amount.
 4. The motor driveas claimed in claim 1, wherein the controller is arranged to compare thedetermined capacitance value to a stored capacitance value and determinea difference between said determined and stored capacitance values. 5.The motor drive as claimed in claim 4, wherein the controller isarranged to compare the determined capacitance value to a plurality ofstored capacitance values or a stored capacitance trend.
 6. The motordrive as claimed in claim 1, wherein the DC bus portion comprises avoltage sensor arranged to produce a sense signal dependent on a voltagebetween the inductor and the DC link capacitor.
 7. The motor drive asclaimed in claim 6, wherein the voltage sensor comprises a potentialdivider connected between the inductor and DC link capacitor, andwherein the node is connected between first and second resistors of thepotential divider.
 8. The motor drive as claimed in claim 7, wherein thepotential divider is connected in parallel across the first and secondconductors.
 9. The motor drive as claimed in claim 1, wherein the notchfilter comprises: a first low pass filter block arranged to remove a DCcomponent of a voltage at the input of the notch filter to produce an ACcomponent; a forward rotation vector block arranged to generate a firstvector having a first element set to zero and a second element set tothe AC component, and to recalculate said vector in a reference framerotating at the resonant frequency of the notch filter, therebygenerating a second vector; a second low pass filter block arranged tofilter a first element and a second element of second vector, therebygenerating a third vector; and a backward rotation vector block arrangedto recalculate the third vector in a stationary reference frame, therebygenerating a fourth vector; wherein a second element of the fourthvector is output as the filtered signal.
 10. The motor drive as claimedin claim 9, wherein the first, second, third, and fourth vectors aretwo-dimensional vectors.
 11. The motor drive as claimed in claim 9,wherein the first element of each vector is an x-component and thesecond element of each vector is a y-component.
 12. The motor drive asclaimed in claim 1, wherein the external load comprises a motor,optionally wherein the motor is arranged to drive an actuator.
 13. Themotor drive as claimed in claim 1, wherein the supply current providedby the inverter is generated by a cascaded current control looparrangement.
 14. A method of operating a motor drive comprising: arectifier circuit portion and an inverter circuit portion having a DCbus portion connected therebetween, said DC bus portion comprising firstand second conductors, wherein an inductor is connected in series alongthe first conductor between said rectifier and inverter circuitportions, and wherein a DC link capacitor is connected in parallelbetween the first and second conductors; and a notch filter having atuneable resonant frequency, wherein a voltage across the DC linkcapacitor is input to the notch filter, the notch filter being arrangedto supply a filtered signal; wherein the method comprises: receiving anexternally supplied AC voltage and to generate a DC bus voltagetherefrom using the rectifier circuit portion; receiving the DC busvoltage and generating an AC output voltage therefrom using the invertercircuit portion; supplying the AC output voltage to an external load;varying the resonant frequency of the notch frequency; modulating asupply current provided by the inverter circuit portion with a probecurrent signal at the resonant frequency; varying the resonant frequencyto a plurality of values across an operational range; measuring anamplitude of the filtered signal; determining which of the plurality ofvalues of the resonant frequency has a maximal amplitude of the filteredsignal associated therewith; and determining a capacitance value of theDC link capacitor from said value of the resonant frequency associatedwith the maximal amplitude.
 15. The motor drive as claimed in claim 1,wherein the controller is arranged to store the determined capacitancevalue in the memory and/or to retrieve the stored capacitance value frommemory.